This invention relates generally to electromechanical relays and more particularly to high voltage relays having fast switching times.
Electromechanical switching relays have long been known and used in the art. The relay operates by passing a current through a wire wound coil to create a magnetic field which is used to close the relay. Thus, relays act as electromechanical switches.
Referring to FIG. 1, a prior art relay 10 is shown. The relay consists of a magnetic core 12, a coil 13, a pivot arm 14, and a conductive contact plate 16. The contact plate 16 has a first end that is hingedly mounted and a second end that moves between an open position as shown in FIG. 1 and a closed position wherein the second end is in electrical contact with a terminal 18. The first end remains in electrical contact with a terminal 20 in both the open and closed positions. An actuation signal is applied across terminals 24 and 26 to actuate the pivot arm which causes the plate to move between the open and closed positions.
A variety of applications require the relay to switch within a predetermined time. For example, one or more relays are used in a heart defibrillator to switch or shunt a stored charge through the patient to restore a normal heartbeat. The defibrillator must be able to apply the charge within a predetermined time or risk damaging a patient's heart. For example, when applying a therapeutic shock to the patient the charge must be applied within a predetermined time after R-wave detection.
An electrical schematic of a defibrillator, indicated generally at 30, is shown in FIG. 2. The series circuit consists of a capacitor C which is used as a charge storage means, a choke inductor L, two patient contact pads 32 and 34 which are applied to the patient, and relays R1 and R2 to shunt the charge. Typically, relays R1 and R2, rather than being separate relays, are actually a double pole double throw (DPDT) switch relay so that R1 and R2 operate in unison.
The switching time of the relay, as measured from the assertion of the actuation signal, is primarily determined by the distance D between the plate and the terminals, as shown in FIG. 1, and the combined mass of the plate and shaft. The obvious solution to decrease the switching time is to decrease the distance D between the plate and the terminals. The minimum distance between the plate 16 and terminals 18 and 20, however, is limited by the voltage applied to terminals 18 and 20, as described below. Defibrillators, which require fast switching times, also require high voltages, e.g., 5 KV. Thus, the conventional relay 10 is not well suited for defibrillator applications.
If the voltage applied to the terminals exceeds the breakdown voltage of the air between the terminals from the contact plate, an arc will form between the terminal and the contact plate. One solution to address the breakdown voltage problem is to enclose the terminals and contact plate in a pressure chamber and replace the air with a pressurized gas which has a higher breakdown voltage than air. An example of such a solution is a relay manufactured by Kilovac of Santa Barbara, Calif., part number KM-13/S31. The Kilovac part uses a sulfur hex fluoride gas under approximately three atmospheres of pressure. The pressure chamber, however, results in considerable added expense over the conventional relay.
Accordingly, a need remains for a relay for switching high voltages having a fast switching time which does not require a pressure chamber.